Isopropylmalate isomerase is an enzyme which catalyzes a reaction on the leucine synthesis pathway to convert 2-isopropylmalic acid to 3-isopropylmalic acid.
Also, an isopropylmalate isomerase large subunit (hereinafter referred to as “ISOM”) is a polypeptide which forms a complex with an isopropylmalate isomerase small subunit to constitute isopropylmalate isomerase.
In recent years, complete genomic sequences have been decoded in various microorganisms such as Escherichia coli and Bacillus subtilis [Science, 277, 1453 (1997), Nature, 390, 249 (1997)], and the nucleotide sequence of a DNA encoding ISOM has also been found.
In microorganisms belonging to the genus Corynebacterium, a gene encoding ISOM was predicted from genomic sequence information in Corynebacterium glutamicum and its nucleotide sequence has been reported (WO 01/00843, EP 1108790).
On the other hand, an ISOM-deficient leucine-requiring mutant has been obtained from a lysine-producing strain of Corynebacterium glutamicum [Appl. Microbiol. Biotechnol., 37, 566 (1992)]. It has been reported in this reference that when this leucine-requiring mutant is cultured in a medium supplemented with a certain concentration of leucine, its growth level is appropriately inhibited, and lysine accumulation increases.
Furthermore, correlation between leucine requirement and lysine production has been examined by using a lysine-producing strain of Corynebacterium glutamicum from which isopropylmalic acid synthase as another enzyme on the leucine synthesis pathway was deficient, and it has been reported that inhibition of the growth level is the highest possibility of the reason for the increase of the lysine accumulation when the added amount of leucine to the medium during the cultivation is limited [Appl. Environ. Microbiol., 60, 133 (1994)].
However, it is not known so far that productivity of L-lysine can be improved by introducing a mutation in the DNA encoding ISOM, without adding leucine to a production medium and without causing significant change in the growth level. In addition, there are no reports describing and suggesting that productivity of not only L-lysine but other many L-amino acids can also be improved by a mutation of ISOM and that introduction of what type of mutation into the DNA encoding ISOM can bring the above-described effect.